In recent years, screens of portable devices, such as smart phones, have become larger. As the screens of the portable devices have increased in size, it has become more and more difficult to control the portable devices with one hand while stably holding the same, which results in a degradation in convenience of use. Furthermore, wearable devices (e.g., smart watches, smart pendants, etc.) that are used in conjunction with portable devices have rapidly propagated. Recently, wearable devices have increasingly attracted a lot of attention since they can be worn on a part (e.g., wrist or neck) of a human body with ease. However, the wearable devices have failed to provide users with benefits other than the usability that they can be worn on a part of a human body, which leads to a gradual decrease in the usability.